Molding compositions based on acetal polymers having a high degree of crystallinity have been in use for many years. They have been used in many applications as, for example, automobile bumper extensions and instrument panels; plumbing supplies, such as valves, shower assemblies, flush tank components, faucets and pipe fittings; tool components; and household and personal products.
These crystalline acetal polymers have excellent physical properties. For many applications, however, improved impact resistance would be highly desirable. Typically, the impact strength of a crystalline polymer is improved by blending it with an elastomer to form shock-absorbing rubbery domains in the crystalline polymer. This approach to impact resistance is most successful when there is a strong interaction between the surface of the rubbery domains and the crystalline polymer. Elastomeric polymers which have strong interactions with a crystalline polymer also are useful for bonding two surfaces of the crystalline polymer and for bonding the crystalline polymer to other materials.
A very limited number of acetal copolymers have been synthesized which are non-crystalline and which are similar enough in chemical structure to the crystalline acetals to interact well with them. One example, commonly assigned U.S. Pat. No. 4,788,258, discloses non-crystalline copolymers derived from trioxane and 1,3-dioxolane with the 1,3-dioxolane content being between about 65 and 75 mol percent of the total monomer composition. These polymers have a glass transition temperature that is less than about -60.degree.C.
Commonly assigned U.S. Pat. No. 4,758,608 teaches noncrystalline acetal terpolymers synthesized from trioxane, 1,3-dioxolane, and formals of monoethylenically unsaturated aliphatic diols. These can be cured with multifunctional crosslinking monomer to produce insoluble, rubbery, non-tacky elastomeric acetal polymers. Commonly assigned U.S. Pat. No. 4,898,925 discloses copolymers having improved elastomeric properties that are made from trioxane, 1,3-dioxolane, and about 0.005 to 0.15% of a bifunctional monomer such as 1,4-butanediol diglycidyl ether or butadiene diepoxide. All of the above non-crystalline acetal polymers form blends with crystalline acetals that show improved impact resistance over unblended crystalline acetals.
Copending, commonly assigned and commonly invented U.S. application Ser No. 406,641 teaches acetal copolymers made from 1,3-dioxolane and 1,3-dioxepane that are elastomeric and that are non-crystalline at temperatures as low as -120.degree. C. and below. These also form blends with crystalline acetal polymers that have improved impact resistance.
Copending U.S. application Ser. No. 636,811, also commonly assigned and commonly invented, discloses acetal terpolymers of 1,3-dioxolane, 1,3-dioxepane, and 4,7-dihy-dro-1,3-dioxepin. These terpolymers are also elastomeric and non-crystalline. After crosslinking or vulcanization, these polymers become non-thermoplastic elastomers.
Most of the compositions described above are thermoplastic materials which readily deform under stress. Crosslinked compositions which have high elasticity and good dimensional stability, that are non-crystalline at temperatures well below room temperature and that are not thermoplastic, would be extremely useful for making high impact acetal blends. The composition taught herein features these properties and is therefore extremely useful for making impact-modified acetal blends.